The present invention relates to an electric power converter having a specific feature in a main circuit configuration thereof for supplying a constant voltage from an AC power supply to a load.
FIG. 10 is a block circuit diagram of a conventional electric power converter for converting AC electric power to DC electric power and for converting the converted DC electric power back to AC electric power.
Referring to FIG. 10, a series circuit consisting of semiconductor switching devices 10 and 11 is connected to an end of an AC power supply 1 via a reactor 40. Diodes 14 and 15 are connected to the semiconductor switching devices 10 and 11 in opposite parallel, respectively.
The semiconductor switching devices 10 and 11, controlled by PWM (pulse width modulation), work together with the diodes 14 and 15 as a rectifier circuit for conducting control and conversion operations to make voltages at capacitors 30 and 31 DC voltages while storing energy in the capacitors 30 and 31 connected in series.
A series circuit consisting of switching devices 12 and 13 is connected to the series circuit consisting of the capacitors 30 and 31. Diodes 16 and 17 are connected to the switching devices 12 and 13 in opposite parallel, respectively. The switching devices 12 and 13 are operated as an inverter through the PWM control to generate an arbitrary stable AC voltage from the smoothed DC voltage, and the generated AC voltage is supplied to a load 6.
A capacitor 32 connected to both ends of the AC power supply 1 works as a filter capacitor. A reactor 41 and a capacitor 33 connected to an input side of the load 6 constitute an LC filter.
An electric power converter substantially identical to the conventional electric power converter shown in FIG. 10 has been described in the following Patent Reference 1.
[Patent Reference 1]
Japanese Patent No. 3203464 (FIG. 1, Paragraphs [0003] and [0004])
The conventional electric power converter shown in FIG. 10 has a so-called double converter circuit configuration in which AC electric power is converted to DC electric power once and the DC electric power is converted back to AC electric power.
FIG. 11 is a diagram for explaining the principle of the circuit shown in FIG. 10. In the circuit shown in FIG. 10, the converter at a side of the AC power supply 1, having the switches devices 10 and 11 and the diodes 14 and 15, works as a rectifier circuit. Therefore, the rectifier circuit can be regarded as a parallel current source 5 through which all the energy necessary for the load 6 flows, as shown in FIG. 11.
Also in FIG. 10, the converter at a side of the load 6, including the switch devices 12 and 13 and the diodes 16 and 17, works as an inverter to supply a certain voltage to the load 6. Therefore, the inverter can be regarded as a parallel voltage source 3 through which all the energy necessary for the load 6 flows, as shown in FIG. 11.
As described above, in the conventional electric power converter shown in FIG. 10 and the double converter type power converter disclosed in Patent Reference 1, all the energy supplied to the load flows through both the converter at the side of the AC power supply 1 and the converter at the side of the load 6. Therefore, loss generated in such a converter becomes high. As a result, the conversion efficiency decreases and running cost increases.
Accordingly, in the present invention, the AC power supply 1 of the double converter is connected to the load 6 in a different way to provide a so-called serial parallel converter configuration, in which the converter at the side of the load 6 works as a series converter. When the voltage of the AC power supply 1 is changed, the series converter compensates only the change in the voltage, and the parallel converter at the side of the AC power supply 1 compensates only the energy necessary for the compensation.
In other words, an object of the present invention is to provide an electric power converter with high conversion efficiency, thereby reducing running cost.
Another object of the present invention is to provide an electric power converter capable of supplying a constant voltage to the load while suppressing the voltage change of the AC power supply 1.